Scala does not apply implicit conversion when required type is derived implicitely

Question

I've probably encountered a version of chained implicit conversions problem (http://docs.scala-lang.org/tutorials/FAQ/chaining-implicits.html). And I cannot figure out how to work it around.

I attached a SSCCE example below which illustrates the issue. Let me add some details here:

1) I want to have a method like the following one:

def chainedTransform[A, B, C](a: A)(f1: F[A, B], f2: F[B, C]): C

It should transform value from type A to type C.

2) It may happen that the transformer F with necessary type parameters is not present in scope, but a satisfying implicit conversion is.

3) The whole approach is demonstrated in the snippet below.

  test("Mutliple conversions") {
    val f1: Function1[String, Int] = (s) => s.toInt
    val f2: Function1[Int, Boolean] = (i) => i > 0
    val f3: Function[Option[Int], Option[Boolean]] = (iOpt) => iOpt map (_ > 0)

    implicit def toOptFun[A, B](f: Function1[A, B]): Function1[Option[A], Option[B]] =
      new Function1[Option[A], Option[B]] {
        override def apply(v1: Option[A]): Option[B] = v1 map f
      }

    case class Test[A](param: A) {
      def transform[B](t1: Function1[A, B]): B = t1(param)
      def chainedTransform[B, C](t1: Function1[A, B], t2: Function1[B, C]): C = t2(t1(param))
    }

    val test1 = Test("123")
    assert(test1.chainedTransform(f1, f2))

    val test2 = Test(Option("123"))
    //toOptFun implicit conversion is applied successfully
    assert(test2.transform(f1) == Some(123))

    //Does not compile: 
    //test2.chainedTransform(f1, f2)
    //  [error]  found   : Int => Boolean
    //  [error]  required: Option[Int] => Boolean
    //  [error]     test2.chainedTransform(f1, f2)
    //  [error]                                ^

    //f3 is exactly what toOptFun(f2) would produce if applied
    assert(test2.chainedTransform(f1, f3) == Some(true))
  }

Any ideas how to make it work?

badcook · Accepted Answer · 2016-04-19 17:59:54Z

2

This is simply because Scala is trying to go for the wrong type for test2.chainedTransform(f1, f2). As the error message suggests, the compiler is looking for an Option[Int] => Boolean when all you have is an implicit conversion to Option[Int] => Option[Boolean].

If you hint to the compiler what you really want with a type annotation, everything will compile.

test2.chainedTransform(f1, f2): Option[Boolean]

As a side note, chainedTransform is reverse function composition (the andThen method on functions) in the specific case of functions and more generally is reverse composition on arrows (the >>> operator for the Arrow typeclass in Scalaz).

edited Apr 19, 2016 at 17:59

answered Apr 18, 2016 at 2:35

badcook

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1 Comment

Łukasz Over a year ago

Are you sure about the Arrow thing? Isn't >>> equivalent of andThen?

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